1. Field of the Invention
The present invention generally relates to a speed control apparatus for a direct current motor. More specifically, the present invention relates to a speed control apparatus for controlling a rotation speed of a direct current motor to a constant speed by controlling a supply of an electric power to a bridge circuit including an armature of the direct current motor the speed of which is to be controlled.
2. Description of the Prior Art
Conventionally a speed control apparatus of a non-contact type referred to as an electronic governor has been put into practical use for the purpose of controlling the speed of a micromotor employed as a prime mover of a tape recorder, for example. FIG. 1 is a schematic diagram of one example of a conventional electronic governor which constitutes the background of the invention. Referring to FIG. 1, a bridge circuit 1 is constituted with resistors 2, 3 and 4 of an armature 5 of a direct current motor. The emitter-collector path of a supply control transistor 7 is connected between one supply point A of the bridge circuit 1 and a direct current voltage source 6. The base of a detecting transistor 8 is connected to one unbalanced voltage detecting point C of the bridge circuit 1. The other unbalanced voltage detecting point B is connected to the emitter of the detecting transistor 8 through a diode 9 for providing a reference voltage. The collector of the detecting transistor 8 is directly connected to the base of the supply control transistor 7. The emitter of the detecting transistor 8 is connected to the ground through a resistor 10, through which a forward current is allowed to flow through the diode 9, whereby the forward drop voltage is utilized as a reference voltage for the transistor 8. A variation of the counter electromotive force in the armature in proportion to a variation of the number of revolutions of the motor is detected by detecting a difference voltage between the unbalanced voltage of the bridge circuit 1 and the reference voltage being applied to the diode 9 by means of the transistor 8. The detected difference voltage is amplified by the transistor 8 and is applied to the base of the supply control transistor 7. The supply control transistor 7 becomes conductive with the degree of conduction associated with the voltage in proportion to the difference voltage being applied to the base thereof, whereby the magnitude of the direct current voltage being applied to the supply point A to the bridge circuit 1 is controlled. Thus, control is made such that a counter electromotive force in the armature 5 may be constant, whereby the direct current motor including the armature 5 is controlled to a constant speed.
With such conventionally known electronic governor as shown in FIG. 1, a voltage drop is caused between the emitter and base of the supply control transistor because the supply control transistor 7 inserted between the direct current voltage source 6 and the bridge circuit 1 operates in a linear region. Accordingly, such conventional electronic governor involved a disadvantage that power consumption is increased due to such voltage drop and the life of a battery becomes shorter in the case where a battery is employed as the direct current voltage source 6. More specifically, since such conventional electronic governor has achieved speed control of a direct current motor by changing a voltage drop across an internal impedance of a supply control transistor 7, a problem was involved that a power loss is unavoidably caused due to such voltage drop.